Bacterial contamination of food products is known to be responsible for spoilage and for the transmission of food borne illness. This problem is particularly important in RTE meats and dairy products which are not normally reheated by consumers prior to ingestion and which are stored for extended times in refrigerators at 2-10° C. An exemplary case is Listeria monocytogenes which is a pathogenic bacterium of particular concern in food products, such as vacuum- or modified atmosphere (MA)-packed RTE meat products, due to its tolerance to refrigeration temperatures, relatively high concentrations of NaCl and anaerobic conditions or in products, such as Fresh Cheese, due to the lack of a heat inactivation step. As a result, a great deal of effort has been expended in attempts to identify natural products that can be safely added to foods for the purpose of inhibiting bacterial growth.
It is well-known to use lactic acid bacteria as starter cultures to induce fermentation of meat products, typically raw salted meat products. The term “starter culture” refers to a preparation containing microbial cells that is intended for inoculating a food matrix to be subjected to fermentation. Starter cultures for meat fermentation are commonly comprised by one or more lactic acid bacteria. The starter culture is intended for providing the desired change in the characteristics of the food matrix during fermentation (e.g. a desired acidification, and certain other sensory and technological parameters). Typically, a starter culture will proliferate during the fermentation process. During the fermentation process the lactic acid bacteria primarily produce lactic acid whereby pH drops to the desired pH-value depending on the culture and the processing conditions (temperature, sugar type/content etc.), and importantly, the sensory properties of the product are distinctly changed.
Antagonistic cultures added to food to inhibit pathogens and/or extend shelf life without changing the sensory properties of the product are termed “protective cultures”. In contrast to starter cultures, protective cultures are not intended to change the sensory properties of the product. Their use or that of their metabolic products (organic acids, hydrogen peroxide, enzymes and bacteriocins) is often referred to as “biopreservation” or “bioprotection” (Castellano, P. and Vignolo, G., “Inhibition of Listeria innocua and Brochothrix thermosphacta in vacuum-packaged meat by addition of bacteriocinogenic Lactobacillus curvatus CRL705 and its bacteriocins”, 2006, Letters in Applied Microbiology. Vol. 43: 194-199). This study demonstrates a bacteriostatic effect on a non-patogenic Listeria species. No bacteriocidal effect to Listeria is reported. Furthermore the “sensoric” evaluation performed was limited to pH measurements.
Besides the establishment of biopreservation as a method to ensure microbiological safety without changing the sensoric characteristics of the product, bioprotective cultures have also been evaluated for their potential of preventing growth of spoilage bacteria (Vermieren, L. et al., “Evaluation of meat born lactic acid bacteria as protective cultures for the biopreservation of cooked meat products”, 2004, International Journal of Food Microbiology, 96: 149-164).
The sensory acceptability of cooked meat products treated with bioprotective cultures may limit the use of the preservation method, and the buffering capacity as well as the content of glucose have shown to be key elements to avoid sensory deviations when applying bioprotective cultures (Vermieren et al., supra).
A re-growth of Listeria monocytogenes has often been observed with the use of bioprotective cultures after an initial phase of inhibition. Re-growth has been ascribed to the development of resistance of L. monocytogenes to the bacteriocins, degradation of bacteriocin molecules with endogenous proteases produced during the growth phase, adsorption of the bacteriocins to the surface of the producer strain, or specific interactions with the food matrix (Dicks, L. M. T. et al., “Use of bacteriocin-producing starter cultures of Lactobacillus plantarum and Lactobacillus curvatus in production of ostrich meat salami”, 2004, Meat Science, 66: 703-708).
The European patent application EP 1.475.432 discloses two Lactobacillus curvatus strains, deposited as PTA-5150 and PTA-5159 and their use for reducing the growth of a microbe in a food or pharmaceutical composition.
The U.S. Pat. No. 4,886,673 discloses three bacteria strains Lactobacillus curvatus DSM 4265, Microccocus varians DSM4263, and Debaromyces hansenii DSM 4260 and their use for preserving meat products. Example 1 describes the use of Lactobacillus curvatus DSM 4265 in the production of cut raw sausage.
The European patent application EP 0.640.291 discloses the use of Lactobacillus curvatus DSM8430 as a starter culture in salami production. It is specifically mentioned that optimal bacteriocin production occurs at temperatures between 15 and 20° C. and that the activity decreases at low temperatures (+4° C.).
Vogel, R. F. et al., (1993, System. Appl. Microbiol., 16: 457-462) discloses the use of Lactobacillus curvatus strain LTH 1174 as a starter culture in salami production. It is specifically mentioned that optimal bacteriocin production occurs at temperatures between 15 and 20° C. and that the activity decreases at low temperatures (+4° C.).
Benkerroum, N. et al. (2005, J. Appl. Microbiol., 98: 56-63) discloses the use of Lactobacillus curvatus strain LBPE as a starter culture in the production of dry-fermented sausages. The fermentation is performed at 30° C., and the drying at 14-16° C. No bioprotective effect was demonstrated at low temperatures.
Mauriello, G. et al. (2004, J. Appl. Microbiol., 97: 314-322) discloses the use of polyethylene films for food packing that are treated with partially purified bacteriocin of Lactobacillus curvatus strain 32Y. In order to produce the bacteriocin this particular strain is grown at 30° C.